Lithium secondary batteries are widely used in various devices such laptops, cameras, camcorders, PDAs, cell phones, iPods and other portable electronic devices. These batteries are also growing in popularity for defense, automotive and aerospace applications because of their high energy density.
Lithium phosphate-based cathode materials for secondary battery have long been known in the battery industry. People have used metal intercalation compound to improve the electrical property of lithium phosphate. One popular intercalation compound is lithium iron phosphate (LiFePO4). Because of its non-toxicity, excellent thermal stability, safety characteristics and good electrochemical performance, there is a growing demand for rechargeable lithium secondary batteries with LiFePO4 as the cathode material.
LiFePO4 has its problems as a cathode material, however. Compared with other cathode materials such as lithium cobaltate, lithium nicklate, and lithium magnate, LiFePO4 has much lower conductance and electrical density. The current invention solves the problem by producing a mixed crystal structure to significantly enhance the electrical properties of LiFePO4.
A mixed crystal can sometimes be referred to as a solid solution. It is a crystal containing a second constituent, which fits into and is distributed in the lattice of the host crystal. See IUPAC Compendium of Chemical Terminology 2nd Edition (1997). Mixed crystals have been used in semiconductors for enhancing light output in light emitting diodes (LEDs). They have also been used to produce sodium-based electrolyte for galvanic elements. The current invention is the first time that a mixed crystal has been successfully prepared for lithium metal intercalation compounds such as LiFePO4. It is also the first time that a mixed crystalline structure has been used as a cathode material for lithium secondary batteries. The new cathode material disclosed in the present invention has significantly better electrical properties than traditional LiFePO4 cathode materials.